A method such as this and an apparatus such as this are known, for example, from the document U.S. Pat. No. 5,097,195, which is incorporated by reference herein.
Gas turbines are being used to an ever increasing extent for the generation of electrical power. In this case, the oil or gas which is burnt in the gas turbine is converted to movement energy and then, via a generator, to electrical energy. The gas turbine and the generator are in this case coupled to one another and form a shaft run. The gas turbine cannot be started until it reaches a specific rotation speed. Below this rotation speed, the shaft run must be accelerated by some other component. This component then still has to assist the gas turbine over a wide rotation speed range.
Nowadays, the generator is itself used for this starting process. In this case, the generator is used as a motor rather than as a generator. However, owing to the high regulation quality required for the starting process and owing to the load on the mains and the generator, the generator must not be connected directly to the mains. It is therefore now normal for the generator to be fed via a static frequency converter (SFC) during this process. In this case, the size of the SFC is governed in particular by the required torque at medium to high rotation speeds. In addition to this starting and acceleration apparatus, a second device is required in order to control the excitation of the generator. The generator field winding is fed either via sliprings or via a brushless excitor. In the case of brushless excitation, a second apparatus may be required for frequency conversion (in this context, see the document U.S. Pat. No. 5,097,195, which was cited above).
In some cases, however, the gas turbine is also started by means of an independent drive acting on the shaft. This can be achieved in various ways. In this case, by way of example, asynchronous motors are used with appropriate frequency converters.
The known solutions for this starting process have the disadvantage of the comparatively high degree of complexity. During acceleration via the generator, the frequency converter has to be matched to the voltage level of the main stator circuit, and switching means must be provided for disconnection in the generator mode. In the case of an independent drive, the mechanics of the shaft run are more complex.